Demand for monitoring cameras has recently increased as a result of commercial establishments such as banks, etc., establishing unmanned installations for providing service at night. Furthermore, in recent years there has arisen a demand for monitoring cameras having a zoom function. For a zoom lens of a monitoring camera, it is especially important that the lens be able to image objects in an environment of low illuminance and also that they have a wide-angle field of view. Furthermore, it has become increasingly desirable that the lens be compact and also provide good optical performance. A two-group zoom lens, as described in Japanese Laid-Open Patent Application H7-35975, for example, is applicable for such a zoom lens. The lens described in this publication is formed of two lens groups, of negative and positive refractive power, respectively, in order from the object side. Such a construction with a first lens group having a negative refractive power has the advantage of a wide angle of view and more easily provides a suitable back focus.
The present invention relates to a wide-angle zoom lens with a field angle of about 120 degrees used in various video cameras or electronic still cameras, etc., with a CCD, and particularly to a small wide-angle zoom lens suitably used in a monitoring camera. In the zoom lens described in the above publication, the brightness at the wide-angle end is about F/2.0; however, an even brighter zoom lens with a brightness of F/1.0 at the wide-angle end has been required as a zoom lens for recent monitoring cameras. In the case of using, in order from the object side, a negative-positive, two lens group construction, if the lens system is to be made brighter it becomes more difficult to obtain good optical performance over the entire range of zooming due to the asymmetry of the wide-angle zoom lens construction, and this results in excessive fluctuation of aberrations.